There's probably a fair few. Child stars especially - many, if not most, of the Grange Hill cast left acting to do other things (some by choice, some because the acting career never took off).
If you made a real fortune out of your celebrity career, of course, it's unlikely you'll need to work at anything that doesn't interest you. But most actors, singers etc - even well-known ones - often aren't making quite the money people suppose. This is especially true of members of "manufactured" pop groups, where the vast majority of the money will go to their managers. I seem to recall one of the girls from Steps say she made more money working in McDonalds than she did in Steps.